1. Field of the Invention
The present invention relates to a robot system provided with a hand which grips a work and makes it move to a work fastening device of a production machine and to an abnormality judgment method which judges abnormality of a work fastened by the work fastening device.
2. Description of the Related Art
In a processing machine which processes a work, an assembly machine which assembles parts with a work, or another production machine, a work is loaded into the production machine, then the production machine processes the work, the production machine assembles parts with it, etc. Further, a work table of the production machine is provided with a work fastening device for positioning and fastening a work at a predetermined position, for example, a clamp device. Further, a robot is widely used in the loading and unloading of a work to and from a clamp device.
In general, a robot uses a hand to grip a work and make it move to a clamp device of a production machine. Further, after the clamp device is used to fasten the work, the hand of the robot releases the work.
Further, the precision of processing a work and the precision of assembly of parts at a production machine are greatly affected by the precision of position of a work fastened using a clamp device. For example, if a machining chip deposits on a work seating surface at the clamp device, the work will be fastened without closely contacting the seating surface. If processing the work in this state, a product different from the target shape will end up being produced. For this reason, when using a clamp device of a production machine to fasten a work, it is necessary to confirm if the work is accurately positioned at a predetermined position.
Japanese Patent Publication. No. 2004-130445A discloses a contact confirmation device which confirms whether a work is in close contact with a seating surface. This contact confirmation device is provided with an air passage which opens at the seating surface at one end, an air supplying means for supplying air to this air passage, and an air flow detecting means which detects the presence of any flow of the air supplied to the air passage. If there is flow of the air at the air passage in the state where air is supplied to the air passage, it is considered that the work is not in close contact with the seating surface where the end of the air passage opens and that air is flowing out from the seating surface. Therefore, the contact confirmation device uses an air flow detecting means to judge the seated state of a work.
In the above-mentioned prior art, the work seating surface at the clamp device is formed so as to come in close contact with the outside surface of a work. However, works have complicated shapes. Also, works are made with relatively loose dimensional tolerances. In such a case, it is difficult to reliably make the outside surface of a work and seating surface closely contact each other. Therefore, sometimes a clearance is formed between the seating surface and the outside surface of the work due to the dimensional tolerance of the work.
If considering the above-mentioned case of using the contact confirmation device disclosed in Japanese Patent Publication No. 2004-130445A as the means for confirming if a work is accurately positioned at the seating surface by a clamp, the following problem arises. That is, the contact confirmation device judges that a work is not in close contact with the seating surface when air flows out from the seating surface. That is, when even a slight clearance is formed between the outside surface of a work and the seating surface, air will flow out from the seating surface; therefore, it will be judged that an abnormality has occurred in the seated state of the work. For this reason, even if the position of the work fastened by the clamp device is kept within an allowable range, if the state of outflow of air is detected, that work will end up being judged as not being accurately positioned.
To deal with the above problems, the method of detecting a position of a work fastened by a clamp device by a sensor has been studied. However, a sensor requires electrical wiring and causes the clamp device to become complicated and more expensive. Further, when processing a work or assembling parts, a sensor in which electrical wiring is required cannot be used if desiring to continuously make the clamp device turn in one direction. This is because the problem of the electrical wiring breaking will arise.